Closed-loop servo control of a DC motor and load system using tachometer feedback is a very common industrial and research application. In the presence of finite shaft stiffness, this exercise becomes involved since flexibility introduces resonance and shaft ringing. To be able to model, predict, and eliminate these high-frequency resonance problems by means of an appropriate controller design, it is essential to have an accurate model for the entire drive system, including the tachometer.

The conventional tachometer model does not recognize any sensor dynamics and treats the tachometer as a simple 'gain'. When high-speed and high-precision motion control is desired, this model proves to be of little use in predicting the system response at high frequencies. This paper presents an accurate tachometer model that takes into account the effect of a weak mutual inductance between the tachometer winding and the motor winding. This magnetic coupling phenomenon leads to noticeable sensor dynamics. We observe that despite being electrically insulated, the tachometer voltage output is influenced by the motor current. The exact tachometer dynamics thus identified is then incorporated in the modeling of a system that has multiple shaft flexibility, and is used for parameter identification and feedback motion control. Predictions using this new model are in excellent agreement with experimental results.